1. Field of the Invention
This invention relates to multiple speed-ratio transmission mechanisms for use in automotive vehicles.
2. Description of the Prior Art
Four-speed automatic transmissions conventionally include multiple planetary gearsets; friction elements, such as hydraulically-actuated clutches and brakes; a torque converter; and damped torque converter lock-up clutch. These transmissions are employed in rear-wheel drive vehicles where the transmission shaft and engine crankshaft are parallel to the longitudinal axis of the vehicle, and in front-wheel drive vehicles where the transaxle and engine crank shaft are parallel to the transverse axis of the vehicle.
A current trend in the automobile industry is to provide five-speed and six-speed automatic transmissions, which conventionally require three planetary gear units and a large number of friction elements to control the gearing that are required in four-speed transmissions. Automatic transmissions having five or six forward speed ratios require greater size, particularly increased length, to accommodate additional planetary gear units and friction elements. Furthermore, automatic transmissions require nonsynchronous gear shifting, which conventionally requires greater use of one-way couplings and more space within the transmission casing, particularly increased length.
These trends toward features that enhance performance of automatic transmissions have produced a need for an extremely compact transmission suitable for use in a rear-wheel drive vehicle and adapted to fit within a space that is small in comparison to the space required for a conventional five-speed or six-speed transmission. U.S. Pat. Nos. 5,106,352 and 5,429,557 describe a multiple speed automatic transmission having gear units comprising constant mesh gear wheels, a double planetary gearset, and various control brakes.